The formation of a zinc phosphate coating also known as a zinc phosphate conversion coating on a metal substrate is beneficial in providing corrosion resistance and also in enhancing the adherence of paint to the coated metal substrate. Zinc phosphate coatings are especially useful on substrates which comprise more than one metal, such as automobile bodies or parts, which typically include steel, zinc coated steel, aluminum, zinc and their alloys. The zinc phosphate coatings may be applied to the metal substrate by dipping the metal substrate in the zinc phosphate coating composition, spraying the composition onto the metal substrate, or using various combinations of dipping and spraying. It is important that the coating be applied completely and evenly over the surface of the substrate and that the coating application not be time or labor intensive.
The zinc phosphate coating compositions are acidic and contain zinc ion and phosphate ion, as well as additional ions, such as nickel and/or cobalt ion, depending upon the particular application. The presence of nickel ions or cobalt ions in such zinc phosphate coating compositions can be objectionable from an environmental standpoint since such ions are hazardous and difficult to remove from wastewater from commercial applications.
In addition, accelerators are often used in such zinc phosphate compositions. A typical accelerator is nitrite ions, provided by the addition of a nitrite ion source such as sodium nitrite, ammonium nitrite, or the like to the zinc phosphate coating composition. Nitrites, however, are not stable in the acidic environment of the zinc phosphate coating composition and decompose to nitrogen oxides which are hazardous air pollutants and which do not exhibit accelerating capability. Therefore, stable one-package coating compositions cannot be formulated; rather the nitrites must be added to the zinc phosphate coating composition shortly before use. Another disadvantage of the nitrite accelerators is that they provide by-products that cause waste treatment problems upon disposal of the spent zinc phosphating solution. It would be desirable to have an accelerator which is stable in the acidic environment of the zinc phosphate coating composition and which is environmentally acceptable.
It is an object of the present invention to provide a zinc phosphate coating composition that avoids the use of nickel and/or cobalt and which still provides excellent coating properties and is stable in an acidic environment of a zinc phosphating solution.
It is another object of the present invention to provide a zinc phosphate coating composition that includes accelerating agents which provide excellent coating properties, are stable in that they will not decompose in the acidic environment of a zinc phosphating solution and which are environmentally acceptable.